1. Field of the Invention
The present invention relates to a lens system suitable for low-cost and high-efficient digital cameras having megapixel resolution over 3 million pixels, particularly to a retrofocus-type lens system.
2. Description of the Prior Art
In digital cameras, the pixel size of the electronic image sensor is generally smaller than that of the image detecting area of the films employed in the traditional cameras. To compete with the traditional cameras in the xe2x80x98wide-anglexe2x80x99 effect, the lens system of digital cameras needs to be scaled down in accordance with the dimensional proportion of the image sensor of digital cameras to the film of the traditional cameras. The decrease in the size of the lens results in the increase in the radius of curvature of the respective surface of the lenses, and thus, the degree of the image distortion increases and the image quality becomes poor. It is therefore necessary to reduce the size of the aperture (F-stop) to eliminate the light around the lens system. The decrease of the F-stop will increase the diffraction limited circle which causes the image to become blurred. Smaller dimensions and bigger aperture are technical trend for designing digital cameras. As the dimension of the digital cameras is gradually scaled down, the space for positioning an optical filter and an image sensor behind the lens system is narrowed accordingly. In this light, a lens system of high resolution with an appropriate space between the lens system and the image sensor behind the lens system is required for modern digital cameras.
The prior art has used a retrofocus-type lens system to maintain the effective focal length of the digital cameras and to increase the back focal length of the digital cameras, as disclosed in U.S. Pat. Nos. 5,796,528 and 6,057,971. In general, the retrofocus lens system consists of front and rear lens groups. The front lens group of the retrofocus system from the most object side has negative refractive power and the rear lens group has positive refractive power. A stop is placed between the front lens group and the rear lens group. The back focal length of this type of lens system is generally extensible to a certain distance which even exceeds the effective focal length of the lens system. In addition, the aperture must be sufficiently enlarged to provide the desired resolution, as disclosed in U.S. Pat. No. 5,513,046.
The conventional retrofocus type lens system of the digital cameras requires a significant number of lenses to meet the market trend for high resolution of image. Accordingly, not only the lens system becomes complicated but also the space for arranging the optical filter and the image sensor is limited. Moreover, the cost and the yield resulting from such complicated lenses are disadvantageous in terms of manufacture. In addition, an unsymmetrical retrofocus lens system will tend to worsen image distortion including field curvature, astigmatism and lateral color in aesthetic sense because of the individual light path of different wavelengths entering into the lens system. Effective reduction of the degree of image distortion has become an important factor for lens design of the digital cameras.
It is therefore an object of the present invention to provide a retrofocus-type lens system to reduce the degree of image distortion.
It is a further object of the present invention to provide a retrofocus-type lens system for low-cost and high-efficient digital cameras having high pixel resolution.
It is another object of the present invention to provide a lens system including three pairs of lens units to reduce the degree of image distortion of the digital cameras.
The lens system in accordance with the present invention comprises three pairs of lens units wherein the first lens unit from the most object side serves as the front lens group with negative refractive power of the retrofocus-type lens system and the second lens unit in combination with the third lens unit serves as the rear lens group with positive refractive power of the retrofocus-type lens system. The lens elements of the first lens unit are meniscus toward the object side. A stop is placed between the front lens group including the first lens unit and the rear lens group including the second and the third lens units. The first lens unit of the front lens group has a negative lens element to the image side. The third lens unit has a negative biconcave lens element to the image side.